Considerable work has been done to improve the construction of cushioning members which utilize fluid filled bladders such as those used in shoe soles. Although with recent developments in materials and manufacturing methods, fluid filled bladders have greatly improved in versatility, there remain problems associated with obtaining optimum cushioning performance and durability. Fluid filled bladder members are commonly referred to as “air bladders,” and the fluid is generally a gas which is commonly referred to as “air” without intending any limitation as to the actual gas composition used.
There are numerous conventional articles of footwear having gas filled cushioning devices in their midsole or outsole. Gas filled cushioning devices are typically referred to as bladders or “air bladders,” and the gas is commonly referred to as “air” without intending any limitation as to the actual gas composition used. One well known type of bladder used in footwear is commonly referred to as a “two film bladder.” These bladders include an outer shell formed by welding the peripheral edges of two symmetric pieces of a barrier material together. This results in the top, bottom and sidewalls of the bladder being formed of the same barrier material. If any one part of a two film bladder needs to be formed of a specific material and/or to a specific thickness, the entire bladder must be formed of that specific material and/or to that specific thickness. Forming a bladder from only two pieces of a barrier material prevents the side, top and bottom walls from being customized.
Closed-celled foam is often used as a cushioning material in shoe soles and ethylene-vinyl acetate copolymer (EVA) foam is a common material. In many athletic shoes, the entire midsole is comprised of EVA. While EVA foam can easily be cut into desired shapes and contours, its cushioning characteristics are limited. One of the advantages of gas filled bladders is that gas as a cushioning compound is generally more energy efficient than closed-cell foam. This means that a shoe sole comprising a gas filled bladder provides superior cushioning response to loads than a shoe sole comprising only foam. Cushioning generally is improved when the cushioning component, for a given impact force, spreads the impact force over a longer period of time, resulting in a smaller impact force being transmitted to the wearer's body. Even shoe soles comprising gas filled bladders include some foam, and a reduction in the amount of foam will generally afford better cushioning characteristics.
The major engineering problems associated with the design of air bladders formed of barrier layers include: (I) obtaining complex-curved, contoured shapes without the formation of deep peaks and valleys in the cross section which require filling in or moderating with foams or plates; (ii) ensuring that the means employed to give the air bladder its complex-curved, contoured shape does not significantly compromise the cushioning benefits of air; (iii) providing regionalized cushioning to an air bladder to account for differences in load corresponding to the anatomical topology of a human foot especially during high loads; (iv) designing air bladders which maximize the cushioning properties of air and are made entirely of flat barrier films; and (v) designing bladders that provide the advantages of complex-contoured shapes and regionalized cushioning and which can be integrated easily into existing midsole manufacturing methods.
The prior art is replete with attempts to address these difficulties, but have only solved one, two or even three of the above-described problems often presenting new obstacles in the process. Most of the prior art discloses some type of tensile member. A tensile member is an element associated with a bladder which ensures a fixed, resting relation between the top and bottom barrier layers when the bladder is fully filled, and which often is in a state of tension while acting as a restraining means to maintain the general external form of the bladder.
Some prior art constructions are composite structures of bladders containing foam or fabric tensile members. One type of such composite construction prior art concerns bladders employing an open-celled foam core as disclosed in U.S. Pat. Nos. 4,874,640 and 5,235,715 to Donzis. These cushioning elements do provide latitude in their design in that the open-celled foam cores allow for complex-curved and contoured shapes of the bladder without deep peaks and valleys. However, bladders with foam core tensile member have the disadvantage of unreliable bonding of the core to the barrier layers. Another disadvantage of foam core bladders is that the foam core gives the bladder its shape and thus must necessarily function as a cushioning member which detracts from the superior cushioning properties of a gas alone. One reason for this is that in order to withstand the high inflation pressures associated with bladders, the foam core must be of a high strength which requires the use of a higher density foam. The higher the density of the foam, the less the amount of available volume in the bladder for a gas. Consequently, the reduction in the amount of gas in the bladder decreases the effectiveness of gas cushioning.
Even if a lower density foam is used, a significant amount of available volume is sacrificed which means that the deflection height of the bladder is reduced due to the presence of the foam, thus accelerating the effect of “bottoming out.” Bottoming out refers to the premature failure of a cushioning device to adequately decelerate an impact load. Most cushioning devices used in footwear are non-linear compression based systems, increasing in stiffness as they are loaded. Bottoming out is the point where the cushioning system is unable to compress any further and is a common failure in shoe soles comprised of foam. Also, the elastic foam material itself performs a significant portion of the cushioning function and is subject to compression set. Compression set refers to the permanent compression of foam after repeated loads which greatly diminishes its cushioning aspects. In foam core bladders, compression set occurs due to the internal breakdown of cell walls under heavy cyclic compression loads such as walking or running. The walls of individual cells constituting the foam structure abrade and tear as they move against one another and fail. The breakdown of the foam exposes the wearer to greater shock forces.
Another type of composite construction prior art concerns air bladders which employ three dimensional fabric as tensile members such as those disclosed in U.S. Pat. Nos. 4,906,502 and 5,083,361 to Rudy, which are hereby incorporated by reference. The bladders described in the Rudy patents have enjoyed considerable commercial success in NIKE, Inc. brand footwear under the name Tensile-Air® and Zoom™. Bladders using fabric tensile members virtually eliminate deep peaks and valleys, and the methods described in the Rudy patents have proven to provide an excellent bond between the tensile fibers and barrier layers. In addition, the individual tensile fibers are small and deflect easily under load so that the fabric does not interfere with the cushioning properties of air.
One shortcoming of these bladders is that currently there is no known manufacturing method for making complex-curved, contoured shaped bladders using these fabric fiber tensile members. The bladders may be of different heights, but the top and bottom surfaces remain flat with no contours and curves.
Another disadvantage of fabric tensile members is the possibility of bottoming out. Although the fabric fibers easily deflect under load and are individually quite small, the sheer number of them necessary to maintain the shape of the bladder means that under high loads, a significant amount of the total deflection capability of the air bladder is reduced by the volume of fibers inside the bladder and the bladder can bottom out.
One of the primary problems experienced with the fabric fibers is that these bladders are initially stiffer during initial loading than conventional gas filled bladders. This results in a firmer feel at low impact loads and a stiffer “point of purchase” feel than belies their actual cushioning ability. This is because the fabric fibers have relatively low elongation to properly hold the shape of the bladder in tension, so that the cumulative effect of thousands of these relatively inelastic fibers is a stiff one. The tension of the outer surface caused by the low elongation or inelastic properties of the tensile member results in initial greater stiffness in the air bladder until the tension in the fibers is broken and the solitary effect of the gas in the bladder can come into play which can affect the point of purchase feel of footwear incorporating a fabric core bladder.
Another category of prior art concerns air bladders which are injection molded, blow-molded or vacuum-molded such as those disclosed in U.S. Pat. No. 4,670,995 to Huang and U.S. Pat. No. 4,845,861 to Moumdjian, which are hereby incorporated by reference. These manufacturing techniques can produce bladders of any desired contour and shape while reducing deep peaks and valleys.
In Huang '995 it is taught to form strong vertical columns so that they form a substantially rectilinear cavity in cross section. This is intended to give substantial vertical support to the cushion so that the cushion can substantially support the weight of the wearer with no inflation. Huang '995 also teaches the formation of circular columns using blow-molding. In this prior art method, two symmetrical rod-like protrusions of the same width, shape and length extend from the two opposite mold halves meet in the middle and thus form a thin web in the center of a circular column. These columns are formed of a wall thickness and dimension sufficient to substantially support the weight of a wearer in the uninflated condition. Further, no means are provided to cause the columns to flex in a predetermined fashion which would reduce fatigue failures. Huang's columns are also prone to fatigue failure due to compression loads which force the columns to buckle and fold unpredictably. Under cyclic compression loads, the buckling can lead to fatigue failure of the columns.
Yet another prior art category concerns bladders using a corrugated middle film as an internal member as disclosed in U.S. Pat. No. 2,677,906 to Reed which describes an insole of top and bottom sheets connected by lateral connections lines to a corrugated third sheet placed between them. The top and bottom sheets are heat sealed around the perimeter and the middle third sheet is connected to the top and bottom sheets by lateral connection lines which extend across the width of the insole. An insole with a sloping shape is thus produced, however, because only a single middle sheet is used, the contours obtained must be uniform across the width of the insole. By use of the attachment lines, only the height of the insole from front to back may be controlled and no complex-curved, contoured shapes are possible. Another disadvantage of Reed is that because the third, middle sheet is attached with connection lines that extend across the entire width of the insole, all the chambers formed are independent of one another and must be inflated individually which is impractical for mass production.
The alternative embodiment disclosed in the Reed patent uses just two sheets with the top sheet folded upon itself and attached to the bottom sheet at selected locations to provide rib portions and parallel pockets. The main disadvantage of this construction is that the ribs are vertically oriented and similar to the columns described in the patents to Huang and Moumdjian, would resist compression and interfere with and decrease the cushioning benefits of air. As with the first embodiment of Reed, each parallel pocket thus formed must be separately inflated.
A prior bladder and method of construction using flat films is disclosed in U.S. Pat. No. 5,755,001 to Potter et al, which is hereby incorporated by reference. The interior film layers are bonded to the envelope film layers of the bladder which defines a single pressure chamber. The interior film layers act as tensile members which are biased to compress upon loading. The biased construction reduces fatigue failures and resistance to compression. The bladder comprises a single chamber inflated to a single pressure with the tensile member interposed to give the bladder a complex-contoured profile. There is, however, no provision for multiple layers of fluid in the bladder which could be inflated to different pressures providing improved cushioning characteristics and point of purchase feel.
Another well known type of bladder is formed using blow molding techniques such as those discussed in U.S. Pat. No. 5,353,459 to Potter et al, which is hereby incorporated by reference. These bladders are formed by placing a liquefied elastomeric material in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized gas is introduced. The pressurized gas forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold to form a bladder having the preferred shape and configuration. The produced bladders typically include a formed seam that is a result of the elastomeric material being forced between the mold halves when the halves are secured together. The seam appears in the center of the sidewalls and is directed outwardly away from the center of the bladder. The seam includes jagged edges and is visible when the bladder is exposed along the midsole of an article of footwear.
Many articles of footwear include at least one opening along their midsole for exposing the sidewalls of a contained bladder. When the exposed sidewalls are transparent, the interior of the bladder is visible. These openings along the midsole are commonly referred to as “windows” and are usually located in the heel and/or forefoot. Examples of such footwear include the NIKE AIRMAX shown in the 1995 and 1997 NIKE Footwear catalogs.
Because the exposed transparent material is vulnerable to being punctured, it must be of a strength and thickness that will resist penetration from external elements. As a result, the requirements of the material used for the exposed sidewalls control the construction, aesthetic and functional characteristics of the entire two film or blow molded bladder. Individual bladder components cannot be customized. Instead, the bladder is formed entirely of the transparent material having the thickness needed to prevent rupturing of the exposed sidewall. This results in the top and bottom of the bladder being formed of the same thick, transparent sidewall material, even if the transparent, puncture resistant material is not needed in these parts of the bladder. Unnecessarily thick top and bottom layers can detract from the overall flexibility of the bladder. Conversely, if certain portions of the bladder, such as the top and bottom surfaces, needed to be made of a thicker material relative to the transparent sidewalls, the transparency and/or flexibility of the sidewalls may be compromised. Using one material for each half of the bladder also prevents the bladder from being customized so different portions of the bladder offer different performance and aesthetic advantages.
Preparing a bladder for being exposed along the length of a sole window can also include expensive and time consuming manufacturing steps. As discussed, a construction seam can result along the sidewalls of a bladder during manufacturing. The seam appears in the center of the sidewall after the bladder has been inflated. The seam includes a thick, rough edge that during the manufacturing of the bladder must be reduced to prevent injury and give the sidewalls a smooth, uninterrupted look. The manufacturing steps taken to reduce the seam line increase the manufacturing time and cost of producing a bladder.
Cushioning system design must meet criteria for both comfort at low loads such as standing, walking, point of purchase feel, and performance at high loads such as running, planting, jumping, pivoting. In analyzing the cushioning characteristics of various devices, it is instructive to view such devices in cross-section. That is, take a visual slice vertically down into the midsole to reveal the cushioning profile of the structure that is to provide the necessary shock absorption and response functions. In prior art cushioning devices, typically any single cross section of the cushioning profile is generally a simple foam core, or a single layer of fluid sometimes surrounded by or encased in foam. This simple profile seeks to balance the low-load—high-load criteria by a compromise to both since a simple cushioning profile provides generally uniform shock absorption and response characteristics along the entire device, but does not provide a complex cushioning profile which can be customized or regionalized to the loads realized at certain points along a bladder.
A problem with manufacturing complex, highly regionalized bladders of two films has been inordinate twisting of the fluid filled part. A non-planar geometry is difficult to integrate into subsequent shoe making processes.
There exists a need for a bladder member which solves all of the problems listed above: complex-curved, contoured shapes; no interference with the cushioning benefits of gas alone; provision of regionalized cushioning that can be coupled to the anatomical features of a foot; and simplified manufacture through the use of flat barrier films and integration into existing midsole construction methods. As discussed above, while the prior art has addressed some of these problems, they each have their disadvantages and fall short of a complete solution.
One object of this invention is to provide a cushioning bladder for footwear with multiple stage cushioning regionalized characteristics constructed of film layers.
Another object of this invention is to provide a bladder for cushioning an article of footwear that can have different materials for its top outer barrier sheet, bottom outer barrier sheet and sidewalls.
A further object of this invention is to provide a method of forming a bladder with inverted seam lines that do not require special treatment during manufacturing.